Transversely excited (TE) pulsed gas discharge lasers commonly include a tangential fan to recirculate lasing gas inside a laser chamber. FIGS. 1a and 1b are cross-sectional end and side views respectively showing the inner structure of a laser chamber 100 in a conventional TE excimer laser (see Akins et al., U.S. Pat. No. 4,959,840, issued Sep. 25, 1990, and incorporated herein by reference in its entirety). A laser enclosure 102 provides isolation between a laser chamber interior 105 and the exterior 110. Typically enclosure 102 is formed by a pair of half enclosure members 112 and 114 (see FIG. 1a), which are coupled together and sealed using an o-ring seal 116, extending along a perimeter of enclosure 102. Laser chamber interior 105 is filled to a predetermined pressure with a lasing gas 108. A pulsed gas discharge is generated in a discharge region 122 by a high voltage pulse applied between a cathode assembly 118 and an anode assembly 120. The pulsed gas discharge typically produces excited argon fluoride, krypton fluoride, or fluorine molecules, which generate laser pulse output energy. The pulse output energy propagates from discharge region 122 through an optical output window assembly 162 (see FIG. 1b). Cathode assembly 118 and anode assembly 120, defining discharge region 122, extend parallel to one another along the length of laser chamber 100.
Recirculation of lasing gas 108 is provided by a tangential fan 140, which rotates about an axis 142 and includes a plurality of substantially parallel straight blade members 144 extending along the length of laser chamber 100 between hub members 146. A typical rotation rate for current tangential fans is on the order of approximately 3800 revolutions per minute (rpm). As shown by arrows in FIG. 1a, the flow of gas 108 is upward through tangential fan 140 and transversely across discharge region 122 as directed by a vane member 152. Lasing gas 108 that has flowed through discharge region 122 becomes dissociated and heated considerably by the pulsed gas discharge. A gas-to-liquid heat exchanger 158 (not shown in FIG. 1b) extending along the length of laser chamber 100 is positioned in the gas recirculation path to cool the heated gas. Other vane members, e.g. vane members 160, direct the flow of gas 108 through heat exchanger 158 and elsewhere along the gas recirculation path. Recirculation cools and recombines lasing gas 108, thereby allowing repetitively pulsed laser operation without replacing lasing gas 108.
There are a variety of current issues relating to laser chamber 100 and its associated components, including, among other things, those described below.
The present tangential fan is difficult and expensive to fabricate. Blade members 144 and hub members 146 are individually stamped and formed from aluminum or another suitable alloy, such as an aluminum/bronze alloy, then dip brazed together to form tangential fan assembly 140, using a braze material typically containing approximately 13 per cent silicon by weight. This is a tedious and labor-intensive process. Because the brazed fan assembly has poor mechanical rigidity, post-machining can cause damage and warpage and is thus difficult or impractical. Therefore it is difficult to achieve precision alignment and critical tolerances. The brazed tangential fan assembly 140 is typically coated with electroless nickel.
Since lasing gas 108 is recirculated and reused, it is important to maintain cleanliness and to prevent contamination of the gas environment within laser chamber interior 105, in order to maximize the pulse energy performance, stability, and working life of lasing gas 108.
Undesirable vibrations in the rotating fan assembly adversely affect bearing life. Reduction of these vibrations will reduce bearing wear and allow the possibility of increasing the fan rotation speed for increased gas flow velocity. Particularly, adverse vibrations are associated with the low present natural vibrational frequency of the rotordynamic assembly, including the fan, bearings, shafts, and magnetic rotor. This low natural frequency is largely attributable to low first and subsequent bending mode frequencies of the fan, due to poor mechanical stiffness.
An aerodynamic buffeting effect has been observed, which, among other things, transmits vibrations to the fan bearings, causing bearing wear and premature failure. Measurements of the frequency of these vibrations suggest that they are caused by gas pressure fluctuations generated each time a fan blade member 144 passes in close proximity to the edge of anode assembly 120. Of importance, the clearance between fan blade members 144 and the proximate edge of anode assembly 120 is particularly close, in order to minimize reverse flow leakage and maximize gas flow efficiency. Previous attempts to reduce aerodynamic buffeting by reshaping the anode assembly have resulted in an undesirable reduction in gas flow velocity by approximately ten or more per cent.
Many applications require a substantially constant laser pulse output energy. However, strong and undesirable fluctuations in pulse output energy have been observed. These fluctuations have been found to be particularly severe at high laser pulse repetition rates.
Accordingly, it would be desirable to fabricate a tangential fan assembly economically, such that the finished fan assembly has improved mechanical rigidity against vibrations. Additionally, it would be desirable to minimize or eliminate potential contaminants from the laser chamber. Further, it would be desirable to minimize or eliminate vibrations arising from aerodynamic buffeting, and to minimize or eliminate pulse output energy fluctuations in a TE pulsed gas laser, particularly at high laser pulse repetition rates.